The present invention relates generally to fuel injection systems for internal combustion engines, and more particularly to a high pressure fuel injection system for diesel engines.
Fuel injection is used in both diesel and gasoline fueled internal combustion engines in view of the precise control of fuel delivery obtainable, optimizing fuel timing and metering with a consequent improvement in engine efficiency. A typical fuel injection system includes a fuel supply tank, a fuel supply pump (low pressure), an injection pump (high pressure), at least one fuel injector and a control system. Pressurized fuel is supplied by the injection pump to a chamber located within the injector, adjacent to a discharge spray nozzle having one or more spray orifices. Such a fuel injector typically includes a spring biased valve at the entrance to the spray orifices and a fuel leak-off conduit which returns leakage fuel to the fuel tank to prevent pressure build up within the spring chamber which would detrimentally affect injector performance.
In diesel engines, a problem exists with particulate emissions which are generated over a wide range of engine speeds. Such particulates are usually composed of either carbonaceous solids, condensed and/or adsorbed hydrocarbons, or sulfates, with the solids component of such emissions correlated to smoke opacity. These particulates are formed in the fuel rich regions within a combustion chamber and are believed to result principally from low pressure fuel injection which produces poor fuel atomization. While over 95% of the particulates formed are subsequently burned as mixing and combustion continues in the combustion chamber, the remaining 5% is discharged in the engine exhaust to the atmosphere.
While increased injection pressures can reduce both particulate emissions and fuel consumption, it is difficult to achieve the proper injection pressures over a wide range of engine speeds and loads. Generally, an injection pump provides a lower rate of fuel delivery at low speeds and a higher rate of fuel delivery at high speeds. Since the typical injector nozzle is a fixed orifice, the varying injection rate results in a variation in injection pressure. At low speed, the injection pressure is low and at high speed it is high. However, both a naturally aspirated and a turbo charged engine need equal or higher injection pressure at speeds and loads lower than rated for good mixing and combustion. The injection system, pump and nozzle orifice size are designed around the maximum pressure and flow quantity required at the maximum rated engine conditions. Since this occurs at the maximum load and speed condition, such injection systems generally operate to provide less than optimal output at other engine speeds and loads, thereby reducing combustion efficiency and increasing the amount of particulate emissions.
One solution to this problem involves modifying the pump to provide higher pressures at low speed conditions. However, this can result in very high pressures at high speed conditions which would overstress the injection system and deteriorate engine performance. A pressure relief device may be provided in the high pressure fuel supply tube to relieve the excess pressure. However, the pump design then becomes more complicated, especially with a multiple injection system. To insure proper fuel distribution to each engine cylinder would require a separate pressure relief device due to the sequential injection requirements of the engine. Such a complex system would significantly increase the cost of an injection system with a probable decrease in reliability. Utilizing a pressure relief device also reduces pumping efficiency by bleeding off varying quantities of pressurized fuel.
Consequently, what is needed in the art is a fuel injection system which provides higher injection pressures over a wide range of engine speeds and loads without overly complicating the injection system or unduly sacrificing pump efficiency.